The present invention relates to a maraging steel with excellent fatigue characteristics and a method for producing the same.
Maraging steel is ultralow carbon-Ni steel or ultralow carbon-Ni-Co steel. It is a steel strengthened by precipitating intermetallic compounds of Ti or Mo, etc. on a matrix of tough martensite. It is tough and high in strength. It also possesses many other advantages not previously available such as good weldability and little change in dimensions by heat treatment. Therefore, maraging steel is used as a structural material in leading-edge technical fields such as space development, ocean development, atomic energy utilization, aircraft, and automobiles. Attempts are also being made to put it to use for a wide range of purposes in diverse fields such as pressure-proof vessels, tools, piston rams, and dies.
However, maraging steel poses the following problems due to its high strength and mechanism of strengthening. Specifically, sensitivity to nonmetallic inclusions in the material increases as the strength rises. The concentration of stress by these inclusions lowers the fatigue strength and tends to create inferior durability.
Therefore, improvement of the fatigue characteristics has been attempted to resolve such problems by melting by vacuum induction melting (VIM), then remelting by vacuum arc remelting (VAR) to raise the degree of cleanness of nonmetallic inclusions by controlled reduction of N and O and thereby to reduce the number of nonmetallic inclusions that serve as the origin of fatigue rupture.
The above technology improved the durability to a certain extent. However, the conditions of use of machinery and constructs have become more rigorous in recent years and demands on the strength characteristics of materials have become increasingly severe.
Further improvement of the durability is also demanded to assure the long-term stability of machinery and constructs. This has led to a demand for the development of maraging steel with superior fatigue characteristics for the construction of machinery. Another problem with the conventional production process was the low productivity and the need for expensive, special vacuum arc remelting equipment since vacuum arc remelting was conducted after vacuum induction melting.
The present invention takes note of these problems and has as its object to propose maraging steel with excellent fatigue characteristics and a production process that makes it possible to manufacture the aforementioned maraging steel easily without vacuum arc remelting. This goal is attained by the present invention described below.
The maraging steel of the present invention has a chemical composition consisting essentially of, in % by weight:
C: 0.01% or less,
Ni: 8-19%,
Co: 8-20%,
Mo: 2-9%,
Ti: 0.1-2%,
Al: 0.15% or less,
N: 0.003% or less,
O: 0.0015% or less.
and the balance Fe and the Ti component segregation ratio and the Mo component segregation ratio in its structure of 1.3 or less each.
The maraging steel of the present invention can suppress the production of nonmetallic inclusions without vacuum arc remelting because it is formed steel with limited N and O contents and components that make it difficult for nonmetallic inclusions to be produced. The maraging steel of the present invention can also suppress the production of a band structure caused by segregation of the components because the Ti component segregation ratio and the Mo component segregation ratio are 1.3 or less each. Generation of the band structure leads to differences in strength at the interfaces of the band structure and the development of fatigue cracks at these interfaces. The present invention can obtain excellent fatigue characteristics by making it difficult for fatigue cracks to develop since the generation of the band structure is suppressed.
The process for producing the maraging steel of the present invention comprises melting a steel of the aforementioned chemical composition, casting the molten steel to obtain a steel ingot, hot forging the steel ingot at a forging ratio of at least 4 for a forged piece, then conducting soaking treatment by keeping the forged piece one or more times in a temperature range of 1100-1280xc2x0 C. for a total hot holding time of 10-100 hours, and then plastic working the forged piece.
According to this production process of the present invention, the steel is formed from the composition that makes it difficult for nonmetallic inclusions to develop, and the hot forging and the soaking treatment (component homogenization and diffusion annealing treatment) are performed under specific conditions. Therefore, the maraging steel can be manufactured easily with the Ti component and Mo component segregation ratios of 1.3 or less each and fewer nonmetallic inclusions. Implementation of this production process also does not require special equipment and provides good productivity because it is not necessary to carry out vacuum arc remelting.
The other maraging steel of the present invention is formed from a steel of the aforementioned chemical composition and contains a nonmetallic inclusion in its structure having a size of 30 xcexcm or less when the size of the nonmetallic inclusion is expressed by the diameter of a corresponding circle when the circumferential length of the nonmetallic inclusion is taken the circumference of the corresponding circle.
This maraging steel makes it possible to limit the content of nonmetallic inclusions since the steel is formed from the composition that make it difficult for nonmetallic inclusions to develop. Making the size of the nonmetallic inclusion be 30 xcexcm or less also makes it possible to obtain excellent fatigue characteristics by eliminating large nonmetallic inclusions that accelerate the expansion of fatigue cracks.
The Ti component segregation ratio and the Mo component segregation ratio in the aforementioned other maraging steel are preferably 1.3 or less each. This makes it possible to suppress the development of a band structure caused by segregation of the components and thereby to further improve the fatigue characteristics.
The process for the production of the other maraging steel of the present invention comprises melting a steel that has the aforementioned chemical composition, casting the molten steel to obtain a steel ingot with a taper Tp=(D1xe2x88x92D2)xc3x97100/H of 5.0-25.0%, a height-diameter ratio Rh=H/D of 1.0-3.0, and a flatness ratio B=W1/W2 of 1.5 or less, taking the diameter of a corresponding circle that has a circumference corresponding to the circumferential length of the top of the steel ingot as D1, the diameter of a corresponding circle with a circumference corresponding to the circumferential length of the bottom of the steel ingot as D2, the height of the steel ingot as H, the diameter of a corresponding circle having a circumference corresponding to the circumferential length of the steel ingot at a location of H/2 as D, and the length of the long side and length of the short side of the steel ingot at a location of H/2 as W1 and W2, respectively, and plastic working the steel ingot to make the size of a nonmetallic inclusion in the steel be 30 xcexcm or less when the size of the nonmetallic inclusion is expressed by the diameter of a corresponding circle, taking the circumferential length of the nonmetallic inclusion to be the circumference of the corresponding circle.
This production process makes the large nonmetallic inclusions separate rapidly by floating from the inside to the top of the steel ingot during casting and makes only small nonmetallic inclusions remain inside the steel ingot. Thus the appropriate plastic working of the steel ingot makes it easy to make the nonmetallic inclusions in the steel be 30 xcexcm or less. Therefore, the maraging steel with excellent fatigue characteristics can be manufactured easily without vacuum arc remelting.
In the aforementioned production process as well, preferably the steel ingot is hot forged at a forging ratio of at least 4 for a forged piece, then submitted to soaking treatment by keeping the forged piece one or more times in a temperature range of 1100-1280xc2x0 C. for a total hot holding time of 10-100 hours, and then plastic working the forged piece to make the sizes of the nonmetallic inclusion in the forged piece be 30 xcexcm or less. This process makes it possible to easily manufacture the maraging steel with the Ti and Mo component segregation ratios in the steel of 1.3 or less each.